1. Technical Field
This invention relates to semiconductor processing of electronic components. In particular, the invention relates to 3-D inductor and transformer elements formed by semiconductor processing.
2. Related Art
Nanoscale CMOS and SiGe BiCMOS processes have recently enhanced the performance of Si-based RFICs up to microwave frequencies. With fT and fmax of the transistor approaching 200 GHz, the requirement for high performance passive elements is the bottleneck in designing state-of-the-art RF circuits. Inductors and transformers play a role in the performance achieved by RF circuits such as voltage controlled oscillators (VCOs), low-noise amplifiers (LNAs), filters, mixers, and power amplifiers (PAs). Among inductor and transformer parameters that limit the performance of an RF integrated circuits are quality factor (Q, where Q=ωL/R, ω=frequency, L=inductance of inductor, R=resistance of inductor)), self-resonance frequency (fsr), and distributed effects. Higher Q inductors and transformers help minimize RF power loss, RF noise, phase noise and DC power consumption of RFIC circuits.
Self-resonance frequency of the inductors and transformers may also affect circuit implementation at higher frequencies. Little work has been done on high frequency transformers with a respectable quality factor and resonance frequency beyond 5 GHz. Some of these approaches may use unconventional fabrication technologies that are difficult to integrate, such as the use of metals like Co.
The quality factor of an inductor or a transformer may be determined by its geometry, the type of interconnect metal (Al, Au or Cu), thickness of the metallization, vertical distance between the underpass/airbridge to the inductor windings, dielectric loss of the substrate (low or high resistivity), and by the selective removal of silicon from beneath the inductor structure. Thicker metallization and removal of substrate material may provide a substantial improvement in Q. Substrate removal has the disadvantage of additional cost of post processing on the inductors and weaker mechanical strength.
High-Q three-dimensional inductors based on stressed metal technology have been recently introduced. Stressed metal technology creates three-dimensional, out-of-plane structures by releasing metal structures from the pre-processed substrate and allowing the metal structures to move, under the material stresses in the metal, in a desired direction. Unlike spiral, in-plane inductors, the magnetic field in these inductors is not perpendicular to the substrate. These recently introduced inductors have shown very high quality factors up to 1 GHz operational frequency due to a reduction of losses associated with eddy currents generated in the Si substrate. At higher frequencies, the quality factor has degraded substantially, due to the magnetic field penetration into the lossy Si substrate.
Therefore, a need exists for three-dimensional microelectronic inductors and transformers exhibiting high Q factors at suitable frequencies.